1. Field of the Invention
The present invention relates to a prism which may be incorporated, for example, in an optical head used for recording/reproducing signals on an optical disk, and a method for producing the prism. The present invention further relates to an optical beam shaping apparatus using the above-mentioned prism for shaping a spatial light intensity distribution (for example, for shaping from an oval distribution to a circular distribution) of a light beam such as laser light, and an optical head device employing such an optical beam shaping apparatus. The present invention further relates to a method for shaping a light beam.
2. Description of the Related Art
FIG. 1 is a schematic cross-sectional view showing an optical beam shaping apparatus disclosed in Japanese Laid-Open Publication No. 62-187321 as an example of a conventional optical beam shaping apparatus. Herein, a refractive index of air is referred to as n0=1.
In the optical beam shaping apparatus shown in FIG. 1, a laser light 2 is emitted from a semiconductor laser 1, transmitted through a collimating lens 3, and is thereby converted into a parallel light 4. The parallel light 4, in turn, is incident on a surface 5A of a prism 5 made of a glass material (with a refractive index of n1) at an incident angle "psgr"1 (wherein "psgr"1 is an angle defined by the incident light 4 and a normal 5Axe2x80x2 to the surface 5A of the prism 5). The incident light 4 is refracted at the surface 5A of the prism 5, and becomes a refracted light 4a having a refractive angle "psgr"1xe2x80x2 with respect to the normal 5Axe2x80x2 and an angle "psgr"2 (not shown) with respect to the incident light 4.
The refracted light 4a is then incident on a surface 5B (or xe2x80x9ca bottom surface 5Bxe2x80x9d which opposes the surface 5A of the prism 5) at an incident angle "psgr"2 (wherein "psgr"2 is an angle defined by the light 4a and a normal 5Bxe2x80x2 to the surface 5B). The refracted light 4a reflects off the surface 5B and becomes a reflected light 4axe2x80x2. An angle between the reflected light 4axe2x80x2 and the incident light 4 is referred to as an angle xcex82xe2x80x2 (not shown).
The reflected light 4axe2x80x2 is incident on the surface 5A at an incident angle "psgr"21 (wherein "psgr"21 is an angle defined by the light 4axe2x80x2 and the normal 5Axe2x80x2), thereby being refracted by a refractive angle "psgr"21xe2x80x2 to the normal 5Axe2x80x2 and becomes emitting light 6. An angle between the refracted light 6 (i.e., the emitting light 6) and the original incident light 4 is referred to as an azimuth angle xcex821xe2x80x2 (not shown).
When the surface 5A of the prism 5 is inclined by xcex11 to the incident light 4, the incident angle "psgr"1 is characterized as follows:
"psgr"1=xcfx80/2xe2x88x92xcex11xe2x80x83xe2x80x83Formula (1)
Further, the following Formula (2) is derived from Snell""s Law at the surface 5A:
sin "psgr"1=n1 sin "psgr"1xe2x80x2xe2x80x83xe2x80x83Formula (2)
Due to this refraction, the incident light 4 is either magnified or reduced by a factor of (cos "psgr"1xe2x80x2/cos "psgr"1) within the refracting plane (i.e., within the plane of the drawing). The azimuth angle xcex82 of the refracted light 4a is given by the following Formula (3):
xcex82=xe2x88x92xcfx802+xcex11+"psgr"1xe2x80x2xe2x80x83xe2x80x83Formula (3)
When the bottom surface 5B is inclined by xcex12 to the incident light 4, the incident angle "psgr"2 is characterized as follows:
"psgr"2=xcfx80/2xe2x88x92xcex12+xcex82xe2x80x83xe2x80x83Formula (4)
Further, the following Formula (5) is derived from the Law of Reflection at the bottom surface 5B:
xcex82xe2x80x2=xcfx80/2+xcex12xe2x88x92"psgr"2xe2x80x83xe2x80x83Formula (5)
The incident angle "psgr"21 of the light 4axe2x80x2 to the surface 5A is given by the following Formula (6):
"psgr"21=xcfx80/2+xcex11xe2x88x92xcex82xe2x80x2xe2x80x83xe2x80x83Formula (6)
Further, the following Formula (7) is derived from Snell""s Law at the surface 5A:
n1 sin "psgr"21=sin "psgr"21xe2x80x2xe2x80x83xe2x80x83Formula (7)
Due to this refraction, the light is further magnified or reduced by a factor of (cos "psgr"21xe2x80x2/cos "psgr"21) within the refracting plane.
The azimuth angle xcex821xe2x80x2 of the emitting light 6 is given by the following Formula (8):
xcex821xe2x80x2=xcfx80/2+xcex11xe2x88x92"psgr"21xe2x80x2xe2x80x83xe2x80x83Formula (8)
Due to the two refractions at the surface 5A, the emitting light 6 is either magnified or reduced by a factor of m within the refracting plane, where m is given by the following Formula (9):
m=(cos "psgr"1xe2x80x2/cos "psgr"1)xc2x7(cos "psgr"21xe2x80x2/cos "psgr"21)xe2x80x83xe2x80x83Formula (9)
By sequentially applying the above-mentioned Formulae (1) through (9), for example, when BK7 is selected as a glass material for forming the prism 5 under the following conditions: an oscillation wavelength of the semiconductor laser 1=0.64385 xcexcm (where n1=1.51425); xcex11=17.59xc2x0; and xcex12=31.34xc2x0, an azimuth angle xcex821xe2x80x2 of the emitting light 6 of 89.9963xc2x0 and a magnification ratio m of 2.501 are obtained. The traveling direction of the emitting light 6 is bent by an angle of about 90xc2x0 with respect to that of the incident light 4 and the beam is magnified about 2.5 times within the refracting plane.
In general, the parallel light 4 derived from the light 2 emitted from the semiconductor laser 1 has an oval spatial light intensity distribution (an oval cross-sectional intensity with an ellipticity of about 2.5). The above-described prism 5 magnifies the spatial light intensity distribution in a direction along a minor axis of the oval distribution, thereby obtaining the parallel light having a circular spatial light intensity distribution (a circular cross-sectional intensity).
However, such a conventional light beam shaping apparatus has the following problems.
A glass material forming the prism 5 always has a wavelength dependency of the refractive index (i.e., xe2x80x9cdispersionxe2x80x9d). Specifically, the refractive index of the light becomes smaller as the wavelength of the light becomes longer. For example, in the case where the prism 5 is made of BK7 under the conditions where an oscillation wavelength of the semiconductor laser 1 is 0.70652 xcexcm, the refractive index n1 of the prism 5 is 1.51243. Under this circumstance, the azimuth angle (emitting angle) xcex821xe2x80x2 of the emitting light 6 is 89.9313xc2x0 which is smaller by 0.065xc2x0 than that in the above-described case where the oscillation wavelength of the semiconductor laser 1 is 0.64385 xcexcm.
Generally, due to variation in the output of the semiconductor laser 1, the oscillation wavelength is momentarily fluctuated several nanometers. When the oscillation wavelength is fluctuated, for example, by 10 nm in the above-described conventional optical beam shaping apparatus which employs the prism 5 made of BK 7, the azimuth angle xcex821xe2x80x2 of the emitting light 6 changes by 0.0104xc2x0.
In the case where the emitting light 6 is focused by an objective lens (e.g., with a focal length of 3 mm) so as to be used in an optical head for recording/reproducing signals in an optical disk, the above-mentioned change in the angle of 0.0104xc2x0 will result in a spot displacement of 0.54 xcexcm. This spot displacement of 0.54 xcexcm is not negligible when the optical head is used for reproducing signals recorded in signal pits of the optical disk on the order of submicrons, and may result in a fatal defect.
A prism of the present invention includes: a first portion made of a first material having a wavelength dependency in a refractive index; and a second portion abutting to the first portion, the second portion being made of a second material having a wavelength dependency in a refractive index which is different from the wavelength dependency in the refractive index of the first material. The first portion and the second portion have shapes such that the wavelength dependency in the refractive index of the first portion and the wavelength dependency in the refractive index of the second portion are substantially cancelled by each other.
The first portion and the second portion may be directly adhered to each other.
The prism may be a reflective-type prism including a reflecting portion within a light path from an external incident light to an emitting light. In such a case, when a surface A denotes a surface of the first portion on which the external incident light is incident, a surface B denotes a joint surface between the first portion and the second portion, and a surface C denotes a surface of the second portion facing the surface B; the light path of the reflective-type prism is such that the external incident light is incident on and refracted at the surface A, the light refracted at the surface A is incident on and refracted at the surface B, the light refracted at the surface B is incident on and reflected by the surface C, the light reflected by the surface C is again incident on and refracted at the surface B, and the light refracted at the surface B is again incident on and refracted at the surface A, and thereby supplied to the outside of the prism as the emitting light.
Preferably, a spatial light intensity distribution of an emitting light is changed from a spatial light intensity distribution of an external incident light.
The wavelength dependency in the refractive index of the second material may be larger than the wavelength dependency in the refractive index of the first material. In such a case, when an emitting light is spatially magnified with respect to an external incident light by the prism, where a plurality of angles made by normals to the surfaces A, B and C with respect to an incident direction of the external incident light on the surface A are xcfx80/2xe2x88x92xcex11, xcfx80/2xe2x88x92xcex12 and xcfx80/2xe2x88x92xcex13, respectively; a relationship xcex1a2 greater than xcex13 greater than xcex11 is satisfied. On the other hand, when an emitting light is spatially reduced with respect to an external incident light by the prism, where a plurality of angles made by normals to the surfaces A, B and C with respect to an incident direction of the external incident light on the surface A are xcfx80/2xe2x88x92xcex12, xcfx80/2xe2x88x92xcex12 and xcfx80/2xe2x88x92xcex13, respectively; a relationship xcex12 less than xcex13 less than xcex11 is satisfied. In the above, xcex11, xcex12 and xcex13 are angles made by the surfaces A, B and C with respect to the incident direction of the external incident light, respectively.
Alternatively, the wavelength dependency in the refractive index of the second material may be smaller than the wavelength dependency in the refractive index of the first material. In such a case, when an emitting light is spatially magnified with respect to an external incident light by the prism, where a plurality of angles made by normals to the surfaces A, B and C with respect to an incident direction of the external incident light on the surface A are xcfx80/2xe2x88x92xcex11, xcfx80/2xe2x88x92xcex12 and xcfx80/2xe2x88x92xcex13, respectively; a relationship xcex13 greater than xcex11 greater than xcex12 is satisfied. On the other hand, when an emitting light is spatially reduced with respect to an external incident light by the prism, where a plurality of angles made by normals to the surfaces A, B and C with respect to an incident direction of the external incident light on the surface A are xcfx80/2xe2x88x92xcex11, xcfx80/2xe2x88x92xcex12 and xcfx80/2xe2x88x92xcex13, respectively; a relationship xcex13 less than xcex11 less than xcex12 is satisfied. In the above, xcex11, xcex12 and xcex13 are angles made by the surfaces A, B and C with respect to the incident direction of the external incident light, respectively.
According to another aspect of the present invention, an optical beam shaping apparatus includes: a light source; a collimating lens for converting light emitted from the light source to parallel light; and a prism which receives the parallel light converted by the collimated lens as an external incident light and supplies emitting light. The optical beam shaping apparatus is used for changing a spatial light intensity distribution between the external incident light on the prism and the emitting light emitting from the prism. The prism has the above-mentioned features.
According to still another aspect of the present invention, an optical head device includes an optical beam shaping apparatus having the above-mentioned features.
According to still another aspect of the present invention, a method is provided for producing a prism including a first portion made of a first material having a wavelength dependency in a refractive index and a second portion abutting to the first portion, the second portion being made of a second material having a wavelength dependency in a refractive index different from the wavelength dependency in the refractive index of the first material. The method includes the steps of: selecting a combination of the first material and the second material; generating a line segment on a coordinate plane which has a refractive index n1 of the first material and a refractive index n2 of the second material as coordinate axes, the line segment illustrating changes in the refractive indices n1 and n2 according to a change in a wavelength of light; generating a plurality of groups of contour lines on the coordinate plane, each of the groups of contour lines illustrating a change of an emitting angle of an emitting light from the prism with respect to the refractive indices n1 and n2, while using, as parameters, a first angle xcex11 a second angle xcex12 and a third angle xcex13 which determine a shape of the first portion and the second portion; finding a combination of the first angle xcex11, the second angle xcex12 and the third angle xcex13 such that a slope of the line segment and a slope of the group of the contour lines at a predetermined emitting angle in the coordinate plane are substantially the same; comparing a magnification or reduction ratio of the emitting light obtained for the found combination with respect to an incident light to a predetermined value; determining a final selection set of the combination of the first angle xcex11, the second angle xcex12 and the third angle xcex13 based on the comparison; processing the first portion and the second portion into shapes determined based on the values of the first angle xcex13 the second angle xcex12 and the third angle xcex13 of the final selection set; and abutting the processed first portion and the processed second portion to each other.
In the abutting step, the processed first portion and the processed second portion may be directly adhered to each other.
According to still another aspect of the present invention, a method is provided for shaping a light beam including a step of applying an external incident light to a prism so as to obtain an emitting light having a spatial light intensity distribution different from a spatial light intensity distribution of the external incident light. The external incident light is incident on a prism which includes a first portion, made of a first material having a wavelength dependency in a refractive index, and a second portion made of a second material to thereby obtain the emitting light having a desired spatial light intensity distribution, the second portion abutting to the first portion and having a wavelength dependency in a refractive index thereof which is different from the wavelength dependency in the refractive index of the first material, the first portion and the second portion having shapes such that the wavelength dependency in the refractive index of the first portion and the wavelength dependency in the refractive index of the second portion are substantially cancelled by each other.
The first portion and the second portion of the prism may be directly adhered to each other.
According to the present invention, a prism includes a first portion and a second portion. Any change in the emitting light angle, which is caused by a wavelength dependency in the refractive index (dispersion) of a material forming the first portion of the prism, is canceled by a wavelength dependency in the refractive index (dispersion) of a material forming the second portion of the prism. As a result, a change in an azimuth angle (the emitting angle) of the emitting light, which otherwise would be caused by fluctuations in the oscillation wavelength of a light source, is suppressed as a whole. Accordingly, no dislocation of a beam spot is caused due to wavelength fluctuations in the light beam emitted from the semiconductor laser as a light source when a light beam (an emitted light beam) emitted from the prism of the present invention is focused by an objective lens.
Thus, the invention described herein makes possible the advantages of (1) providing a prism in which the adverse influence of the wavelength dependency in refractive index (dispersion) of a glass material for forming the prism is cancelled, and a method for producing such a prism; (2) providing an optical beam shaping apparatus using the above-mentioned prism, in which a change in the emitting light angle is suppressed without being affected by the fluctuations of oscillation wavelength of a semiconductor laser as a light source, and a method for shaping an optical beam; and (3) providing an optical head device using the above-described optical beam shaping apparatus and the above-described method for shaping the optical beam.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.